Analyzing a sample remotely can entail transmitting light to and from the sample over one or more optical fibers. However, the transmitting light may interact with the optical fibers to impair or complicate the analysis. A number of approaches have been proposed to address competition between light-matter interactions occurring within the optical fibers and light-matter interactions occurring within the sample.
One approach entails transmitting source light from a light source to the sample over a dedicated light-delivery optical fiber and transmitting sample light (stemming from light-matter interactions occurring at the sample) from the sample to a detector over a different, dedicated light-collection optical fiber. An optical system located at the distal end of the optical fibers transfers light to and from the sample. The distal optical system attenuates light due to light-matter interactions occurring within the light-delivery fiber to reduce corruption of the source light associated with transmission from the light source to the sample. The distal optical system may further block the source light from transmitting on the light-collection optical fiber. Excluding the source light from transmitting on the light-collection optical fiber can avoid the source light from causing light-matter interactions in the light-collection optical fiber that would mix with and potentially be difficult to differentiate from the sample light.
While providing segregated light-delivery and light-return paths may offer advantage to some applications, other applications would benefit from delivering and collecting light over one, common optical fiber. However, with conventional technology, the material analysis may struggle to differentiate between light-matter interactions occurring in the bidirectional optical fiber and light-matter interactions occurring in the sample. That is, the source light traveling towards the sample can interact with material of the bidirectional optical fiber and produce light traveling away from the sample that mixes with the sample light that is also traveling away from the sample. For example, in the case of Raman analysis, laser light traveling towards the sample can interact with silica of a conventional bidirectional optical fiber to produce “silica Raman” light and/or fluorescence that may be more intense than the Raman scattered light from the sample (the “Raman light”) that is the basis of the material analysis.
Most of the conventional proposals for conducting Raman analysis using a single optical fiber for light delivery and light collection have limitations. One conventional proposal entails conducting Raman analysis outside the fingerprint spectral region of the Raman spectrum, where silica Raman is weak relative to Raman scattering of the sample. However, Raman scattering generally is also weak outside the fingerprint region, resulting in noise. Further, outside the fingerprint region, distinguishing features between or among competing spectra of a sample typically diminish. Accordingly, conducting Raman analyses outside the Raman fingerprint region can be challenging, particularly when the sample has numerous chemical constituents that the analysis seeks to distinguish. The Raman fingerprint region is typically considered to be about 500 cm−1 to about 2000 cm−1 (wavenumbers) for organic molecules.
Another conventional approach involves a crystal optical fiber transmitting laser light from a laser to a sample and transmitting Raman light from the sample to a detector. Periodically spaced structures in the optical fiber direct the laser light and the Raman light via constructive and destructive interference. One drawback to most conventional approaches for using a crystal optical fiber for transmitting source light and sample light bidirectionally concerns the crystal optical fiber imposing its own spectral signature on the sample light. In other words, conventional crystal optical fibers can have transmission characteristics that deviate across the Raman fingerprint region. For example, spikes, peaks, valleys or undulations in a crystal optical fiber's transmission profile can distort or otherwise impact a Raman spectrum of the sample. Another drawback concerns silica Raman generated as the laser light interacts with the periodically spaced structures and/or propagates between periodically spaced structures. Accordingly, materials and optical characteristics of conventional crystal optical fibers can impose problematic background, artifacts, and/or interference on a spectrum.
Another issue facing many conventional technologies concerns coupling light between the distal end of one or more optical fibers and a sample when the media between that distal end and the sample scatters light, absorbs light, or has challenging light transmission characteristics. In this situation, such challenging media may produce a signal that interferes with the signal of interest from the sample. As another potential problem, the challenging media may diffuse or attenuate the light traveling towards or away from the sample. The challenging media may smear spatial resolution, distort acquired images, or otherwise disturb, confuse, complicate, or confound an analysis.
In view of the foregoing discussion of representative deficiencies in the art, a need exists for improved technologies for analyzing materials over optical fibers and for improved optical fibers and optical waveguides and associated coupling optics. Need is apparent for a system that can transmit source light from a source towards a sample and transmit sample light from the sample towards a detector while avoiding or mitigating interference from lightmatter interactions occurring during transmission. Further need exists for improved technologies for analyzing materials remotely via Raman analysis, including in vivo, in situ, in vitro, and/or ex vivo Raman spectroscopy. Additional need exists for improved technologies for conducting optical coherence tomography (“OCT”), surface enhanced Raman spectroscopy (“SERS”), near infrared (“NIR”) analysis, ultraviolet (“UV”) or visible (“VIS”) spectroscopy, UV resonant Raman spectroscopy (“UVRRS”), imaging, surface plasmon resonance (“SPR”), coherent anti-Stokes Raman scattering (“CARS”), anti-Stokes Raman, Fourier transform Raman (“FT-Raman”), elastic scattering, laser Doppler shift, hyperspectral imaging, surface enhanced resonance Raman spectroscopy (“SERRS”), stimulated Raman, spontaneous Raman, spatially offset Raman spectroscopy (“SORS”), hyper Raman, or some other appropriate analytical technique or instrumentation that utilizes light. Need further exists for light-based characterization of cardiovascular or cardiac tissue over a catheter. Need also exists for an improved system for coupling light into or out of one or more optical fibers. Need exists for technology that can transmit light through optically challenging media, including in, across, or through blood and other biological materials. Need exists for a technology that can form an optical path through media having poor light transmission characteristics. A technology addressing one or more such needs, or some other related shortcoming in the art, would benefit photonics, including promoting in vivo analyses and facilitating new applications.